I saw some posts on the internet about AWS Lambda user progress meter but there were no specific answers. One answer was that "you can use SNS" but I think I solved it using SQS:
Lambda 1 receives the uploaded file, parses the multipart request in a non-best practice way (maybe I should use chalice here) and writes the file to S3. Lambda 2 listens to the S3 event and starts processing the file to transform Resx/JSON/YAML to database fields in MariaDB which takes about one minute. While doing the loop, lambda 2 writes to SQS the progress. Lambda 3 reads the progress from S3 and can report % processed file to the user.
I only created the API but I think that this will work for a good web front-end. Do you agree?